There are two categories of known principles for improving the conveyance of real-time streams (or streaming) in an unstable environment (such as the Internet or wireless links). A first principle consists in acting on the transport protocol itself as described in the US patent document 2006/0198300A1 (by EPSON Research and Development Incorporated, “Multi-channel TCP connections”).
This patent document describes an example of operation of the principle where multiple TCP connections (Multi-TCP connections) are set up between two remote apparatuses (such as tunnel end-points or Internet gateways) on which the application streams will be transmitted.
Starting from the observation according to which the TCP protocol is reliable but does not ensure a constant real-time bit-rate during losses, the idea consists of the use of an aggregate of TCP connections in order to obtain a more regular total bit-rate. The present patent document discloses a technique for selecting one of the TCP carriers to transmit each passenger packet as a function of the state of congestion of the TCP connections.
When a TCP connection undergoes a great reduction of its connection window (named “cwnd” according to the TCP protocol), it means that there is a momentary congestion. Then, another TCP connection with a greater available congestion window is chosen. Thus, if a packet is lost and is being retransmitted on the first connection, the following packets will not be subjected to this congestion on another TCP carrier.
The advantage of this approach pertains to the reliability of transport. However, the passenger stream conveyed through multiple TCP connections is nevertheless disturbed. There are no longer losses but delays in delivery for the packets retransmitted on each of the TCP connections.
According to the multi-TCP principle, the large number of TCP connections entails an equivalent increase in the probability of loss or retransmission. Thus, the de-sequencing of the data upon arrival (not described in the patent document mentioned here in) calls for a latency that is all the longer in order to be corrected.
Thus, the solution of this patent document provides a partial response to the problem of data loss but does not provide for regular delivery of RTP streams without interruption.
A second principle consists of action on the content transported as described in the RFC-2198 standard “RTP Payload for Redundant Audio Data”.
An FEC (forward error correction) type mechanism is a mechanism for protection against errors used during data transmission. The sender adds redundancy in order to enable the intended recipient to detect and correct at least one part of the errors. This prevents retransmission and therefore provides for bandwidth savings and even ensures transmission in certain situations where there is no return channel.
The RFC-2198 standard lays done the principle according to which redundant copies of audio data elements are transmitted in a single RTP stream in order to correct disturbances related to losses in the transportation of the RTP stream.
Thus each RTP packet contains a piece of audio data for a same time slot and a (more compressed) copy of the audio data of a previous time slot. This enables an approximate recomposition of the samples lost from the decoding of the next packet.
This solution requires substantial over-occupation of the available bandwidth so as to convey data in duplicate, and is therefore better suited to WLAN (wireless local area network) wireless environments subject to losses of data as well as to WAN environments where the non-delivery of a packet results from a phenomenon of congestion on the path.
Thus, in this second approach, the redundancy of the information only aggravates the phenomenon of congestion in the WAN global network where bandwidth is limited.